After much trial and error I was able to configure PCem 13.1 shaders to nearly perfectly emulate the shadow mask CRT raster of an old IBM 8515 VGA. See the attached screenshot for real vs emulated comparison (captured on a 4K UHD monitor). I am using CRT-Lottes.glsl shader from retroarch with the parameters specified in the attached PCem configuration file (using Model 30-286 in my example), if anyone is interested.